DC-DC converters are switched mode power supplies (SMPS) used in a variety of circuits to provide a DC output signal by converting a DC input signal. The input and output signals can be of the same or opposite polarities. A variety of different DC-DC converter topologies are available, including isolated and non-isolated supplies. Non-isolated SMPS converters include buck, boost, buck-boost, Cuk and other types. Conventional buck-boost DC-DC converters can operate to deliver output voltages above or below the voltage of the input signal, in contrast to buck converters in which the output voltage is less than or equal to the input voltage, or boost converters where the output voltage is greater than or equal to the input voltage.
However, switching transistors in conventional buck-boost converters are exposed to voltages equal to the sum of the input voltage and the absolute value of the output voltage. In particular, the voltage swing on a switching node, and the voltage across the inductor of a standard two transistor inventing buck-boost converter, are higher than the switch and inductor voltages of buck or boost converters.
The inductor voltage is approximately equal to the input voltage during the on time of the output transistor (the Ton phase), and is approximately equal to the output voltage during the output transistor off time (the Toff phase). Higher inductor voltages lead to larger ripple currents that cause higher inductive switching losses. At the same time, the buck-boost converter switching transistors must be larger than buck or boost converter transistors to accommodate the higher voltage amplitudes. As a result, the larger switching transistors lead to higher switching losses. Thus, while buck-boost converters generally provide more flexibility with respect to output voltage levels, conventional buck-boost converters suffer from lower output current to average inductor current ratios and higher inductor losses compared to buck or boost converters. Consequently, conventional buck-boost converts have lower efficiency, particularly for extreme duty cycle operation.